HF Antennas

In this section you will find some HF Antennas that are easily constructed.....

Cobra 160 -10 meter

BUILD THE "COBRA" ANTENNA
By Raymond A. Cook W4JOH
Taken from and re-edited from a project in
73 Amateur Radio Today
June, 1997


The original Cobra antenna designed by W4JOH can be built as an all band hf antenna covering either 160 thru 10 meters or 80 thru 10 meters and is built using standard insulated wire of about 14 gauge and fed with 450 ohm ladder line down to the shack into a tuner. It got it's name from the S shaped configuration of it's multi-conductor elements. It performs on it's primary and harmonic operating frequencies as a standard ladder-line fed doublet.

The close spaced wire elements on each leg introduces two added resonant responses BELOW the antenna's fundamental operating frequency.

The 140 foot version (80 meters) in picture also resonates at about 2.8mhz and also on 160 meters. A standard dipole at 1.9mhz is about 246 feet total compared to 140 feet in the Cobra!. This fact alone makes this an ideal antenna for restricted space on the TOP BAND!

The half sized version, 73 foot (40 meter) also covers 60 and 75 meters!
All band operation has been reported in the original article to be excellent! (With a tuner of course).
This antenna design extends the coverage compared to a G5RV both in bands and performance. On its primary and harmonic operating frequencies, tests show no discernible difference in signal strength between a Cobra and a regular full-sized doublet or dipole.
On its sub-bands bands where the Cobra is physically "short", efficiency is somewhat lower than for a full-sized dipole.

If you do the math, you will see that there is actually about 420 feet total wire across the top of the antenna on the 80 meter version, (210 feet per side), and about half that on the 40 meter version. The flattop and lead-in length were strictly determined by the physical limitations of the antenna farm and this project is a result of those limitations and the idea of compressing or folding the wire back on itself to fit the antenna farm. (No formulas were given in the article), but they seem to be this:

1/2 of total known length / frequency = multiplier for formula below:

210 / 3.750 (band center) = 56 (unknown multiplier)

So 56 X 3.750mhz (band center) = 210 feet per side. Which is exactly what he used per side.

Editor's note: "This formula is mathematically correct in solving for the unknown assuming the lowest band center frequency was used, but may not be what was used in the original antenna experimentation if any formulas were used at all! The original author, W4JOH, may have arrived at the lengths strictly by experimentation and found them to work well."....N4UJW

Keep in mind that there are actually 3 conductors connected in series per side and folded back on each other..... or another way of saying this is that there is one continuous length of 210 feet per side in the 80 meter version connected to one side of the ladder line and the same on the other half. Because the Cobra antenna is a balanced load, it is recommended to install a 4:1 current-style balun at the station end of the feedline (many external tuners provide a built-in balun). Ladder-line feed may have to be trimmed for lowest SWR, but using about a 100 foot length seems to make for easier tuning on all bands.
 Extra feedline should be suspended in loose coils and not in a tight roll.

A 4:1 balan possibly could be installed at the antenna, then fed with 50 ohm coax to the radio, BUT, it is not known if this would upset any characteristics of the original design.

Experiment!

Raymond is quoted from the article..... "Some of our more skeptical, and perhaps knowledgeable, friends have expressed concern about impedance, power rating, wave-cancellation, etc.  All that we can offer as an answer is the slogan used for many years by the Packard Motorcar Company.
Ask the man who owns one." ........W4JOH

An Attic Coaxial-Cable Trap Dipole for 10, 15, 20, 30, 40, and 80 Meters

  By John DeGood, NU3E

nu3e@arrl.net

Original Article at:

http://degood.org/coaxtrap

Thanks John, NU3E

Abstract

A coaxial-cable trap dipole antenna installed in the attic provides a surprisingly effective solution to HF operation on the 10, 15, 20, 30, 40, and 80 meter amateur bands at a QTH with restrictive covenants that prohibit outside antennas.

Restrictive Covenants

When we purchased our first home in 1980 amateur radio antenna siting was a top selection criteria. But when a job change in 1995 required relocation, my XYL announced that it was "her turn" to choose our new QTH, and amateur radio was not on her priority list! She chose a beautiful new home in a development with excellent amenities for raising our family, but it came with restrictive covenants that prohibit any outside antenna other than a "small antenna for television reception." I feared my HF operating days might be over.

My early HF operating attempts at the new QTH were not encouraging. The landscaping on our new lot consisted of ornamental trees and shrubs that were barely taller than myself. I tried a full wave horizontal loop of nearly invisible small gauge wire which circled the house hanging below the aluminum gutters, but its performance was disappointing and it caused severe RFI problems, forcing me to limit operation to QRP power levels. I next tried an inverted vee using the same stealthy wire, with the peak supported by the house and the ends supported by ornamental shrubs at corners of the lot. It performed as a classic "cloud warmer" that worked for local contacts but it was a lousy DX antenna. And the low height of the shrubs that served as end supports made mowing the lawn look like I was practicing for a limbo contest!

Attic Installation

One day while staring at our lot I considered the attic as a possible antenna location for the first time. Some of the positive attributes were:

• height - the roof ridge on our 2-story home is almost 30' above ground level. This is several times higher than any other object on our property, and is high enough (minimum 1/2 wavelength height) for a horizontal dipole to have a reasonably low angle of radiation on the 10, 15, and 20 meter bands.
• stealth - any antenna in the attic would be completely hidden, so it would not violate the restrictive covenants.
• freedom from environment - an outside antenna must withstand the abuse of wind, moisture, ice, UV, birds, squirrels, etc., but the attic provides protection from all these failure mechanisms.
• simple construction - without environmental stresses to worry about, antenna mechanical and electrical construction is greatly simplified!
• ease of erection and modification - as long as one is careful not to fall through the ceiling, the attic provides easy access to the antenna in almost any weather. However, summer work in an attic is best performed on overcast days, at night, or in the early morning hours.

But there were negative attributes, too:

• RFI - an attic antenna may interfere with household electrical and electronic systems due to its proximity.
• interactions with nearby objects - electrical wiring, plumbing, ductwork, and other construction materials may adversely interact with an attic antenna.
• reduced bandwidth - if a shortened length antenna is chosen to accommodate the space limitations of an attic installation, it can reduce the SWR bandwidth.
• RF exposure - because of the proximity to residents of the house, be sure to conduct an RF safety evaluation.
• fire safety - be certain your antenna design and construction are appropriate for the power level you intend to use. You don't want a trap or end insulator to catch fire in your attic! Both my experience and the amateur literature suggest that the antenna described here should safely accept 100 W if carefully constructed and installed.

Our attic consists of 2x4 wood truss construction. The ridge of the main span is approximately 44 feet long with a non-metallic ridge vent. The roofing material is asphalt composition shingles. The siding and soffits are vinyl. The roof is a 12" pitch (i.e. a 45 degree angle) which results in a tall ridge height. The plumbing vent stacks are PVC plastic. The only significant metal objects in the attic are various runs of electrical wiring that service the 5 smoke detectors and 3 ceiling fans installed in the second floor ceiling, and two lengths of flexible ductwork. As attics go, I consider ours is very amenable to the presence of an amateur HF antenna.

After consideration of the alternatives, I chose to construct a trap dipole antenna in my attic using coaxial-cable traps. I desired multiband capability, and selected a single trap dipole over parallel dipoles or a hybrid design consisting of 2 or more trap dipoles in parallel. Parallel dipoles are more difficult to tune due to interactions between the elements. Also, antenna traps function as loading coils below their resonant frequency and result in a shortened antenna: by using a single dipole design with multiple traps I was able to fit 40 meter coverage comfortably along the 44 foot main ridge of my attic roof. I included 80 meter coverage by adding a pair of 40 meters traps and making a right angle bend at each end of the attic, continuing the 80 meter segments down a few inches below and parallel to the slope of the roof. Since most of the current, and hence most of the radiation, comes from the central portion of a half-wave dipole, this is a reasonable compromise.

An antenna tuner could also be used to accomplish multi-band operation in conjunction with a non-resonant antenna. I prefer a resonant trap dipole design for the following reasons:

• Non-resonant antennas present high SWR which results in large losses when coaxial cable feedline is used. These losses can be reduced to an acceptable level by using open wire feedline. However, it would be very difficult to route open wire feedline between my operating location and the attic, so I wanted to use a coaxial cable feedline.
• Non-resonant antennas can produce a complex radiation pattern with sharp peaks and nulls, e.g. when operating at 10, 15, or 20 meters on an antenna that is an electrical half wavelength on 80 meters. The resonant antenna I constructed produces the characteristic radiation pattern of a half wave dipole on every band, so one need not worry about missing a contact because the other station happens to lie in a null of a complex antenna pattern.
• Non-resonant antennas require tuning when changing bands.
• The series trap dipole construction results in a significantly shortened antenna vs. a non-resonant wire dipole. This is a significant attribute because of space limitations in typical attics. Adding loading coils to a non-resonant wire dipole could achieve a similar result, however.

Coaxial-Cable Trap Construction

The clever use of coaxial-cable to construct antenna traps was first described in the amateur literature by Johns in 1981.[1] Coaxial-cable traps are inexpensive, easy to construct, stable with respect to temperature variation and capable of operation at surprisingly high power levels.[2,3] The traps used in this antenna are based on the "optimized" design graphs derived by Sommer.[4]

Coaxial-cable antenna traps are constructed by winding coaxial-cable on a circular form. The center conductor of one end is soldered to the shield of the other end, and the remaining center conductor and shield connections are connected to the antenna elements. The series-connected inner conductor and shield of the coiled coaxial-cable act like a bifilar or parallel-turns winding, forming the trap inductor, while the same inner conductor and shield, separated by the coaxial-cable dielectric, serve as the trap capacitor.

The resultant parallel-resonant LC circuit exhibits a high impedance at the resonant frequency of the trap and effectively disconnects everything after the trap from the antenna. Any inner traps (which are operating below their resonant frequency) function as loading coils and shorten the overall physical length of the antenna.

I constructed my traps using good quality RG-58/U coax scavenged from a discarded 10BASE-2 Ethernet cable. PVC couplings were used for the trap forms: PVC couplings are very inexpensive, readily available in useful diameters, and can be purchased individually, whereas PVC pipe is usually sold only in 10 foot lengths. 14 gauge solid wire was used to form "bridle wires" for electrical termination of the coax and electrical and mechanical termination of the antenna wire elements.

Coaxial-cable traps must be "tuned" before use. The coax turns were spread slightly until the desired resonant frequency was reached, as measured by a dip meter whose signal was monitored on a nearby calibrated receiver. After adjustment the coax turns were secured by coating with lacquer (I used Deft® brand left over from a furniture finishing project.)

Here are several important points to keep in mind if you attempt to build coaxial-cable traps:

1. The outside diameter of the trap form is a critical dimension. I used Schedule 40 PVC couplings as trap forms (NOT Schedule 40 PVC pipe!) Note in Table 1 that the nominal size of the PVC couplings represents the PVC pipe size the coupling is designed to join, which is significantly smaller than the outside diameter of the coupling.
2. Coaxial-cable traps have a relatively high Q, which results in a relatively sharp frequency resonance. You must adjust (i.e. tune) the traps or the antenna will not work properly, as traps can't do their job if they don't resonate (i.e. become a high impedance) at the correct frequency.
3. If you don't have a dip meter, you can use an HF antenna analyzer, such as those made by Autek or MFJ, to adjust the trap resonant frequency using the procedure in the section titled TRAP FREQUENCY MEASUREMENT at http://www.autekresearch.com/uses.htm. With either a dip meter or antenna analyzer you will get the most accurate result by using the minimum coupling between the trap and the measuring instrument which produces a discernable dip.
4. Unlike traps made from a discrete inductor and capacitor, coaxial-cable traps at resonance, i.e. in their high impedance state, exhibit a different amount of end loading depending on which end faces the center of the antenna. Either orientation works, but to maintain dipole symmetry trap pairs should always be installed symmetrically. I use the easy to remember rule, "Connect center conductor of trap coax toward center of antenna."

Table 1. Specifications of the traps used in this antenna

band	design frequency	trap form	coax length	coax turns	actual frequency
10 meters	28.85 MHz	1.375" OD (3/4" PVC coupling)	20.25"	4	28.5 MHz, 28.7 MHz
15 meters	21.225 MHz	1.375" OD (3/4" PVC coupling)	26"	5.25	21.1 MHz
20 meters	14.175 MHz	1.625" OD (1" PVC coupling)	35.5"	6	14.2 MHz
30 meters	10.125 MHz	2.0" OD (1.25" PVC coupling)	46.25"	6.5	10.12 MHz
40 meters	7.15 MHz	2.25" OD (1.5" PVC coupling)	61"	7.75	7.15 MHz

The 10 and 15 meter traps, wound on 3/4" PVC pipe couplings.

The 20 meter traps, wound on 1" PVC pipe couplings.

The 30 meter traps, wound on 1.25" PVC pipe couplings.

The 40 meter traps, wound on 1.5" PVC pipe couplings.

Trap Connections

I used this simple method to connect the traps to the antenna wire elements. I soldered a short (approximately 2") wire pigtail to the bridle wire on each end of the trap. Then the antenna wire was looped through the trap bridle wire and secured to the pigtail using an electrical wire nut. This made trimming the lengths of the antenna elements easy, as the connections could be readily disassembled and no soldering in the attic was required. When the antenna trimming was complete I used a nylon cable tie to secure the antenna wire loop to the pigtail to strain relieve the connection.

I used 14 gauge stranded household electrical wire for the antenna elements. This wire is very inexpensive when purchased in 500 foot spool quantities at home centers. The insulated jacket causes the wire to have a velocity factor somewhat lower than that of bare copper wire. This is a beneficial attribute for an antenna intended for limited space use such as in an attic, as it results in a shorter overall length.

Center and End Insulators

The antenna center insulator was constructed from a piece of scrap Plexiglas® stock[*]. The center of a half-wave dipole is a current feed point so just about any insulating material will work here. Plastic cable ties are used to secure the antenna elements and the RG-58/U feedline to the insulator. A rope attached to the topmost hole is used to support the antenna center. The rope is approximately twice the height of the attic. It passes through a screw eye secured at the peak of the attic which functions like a pulley, allowing the antenna center to be easily raised and lowered.

I used "real" pulleys at the ends of the attic where the 80 meter segments were bent 90 degrees to fit within the available space. The insulated 14 gauge wire rolls easily over the "real" pulleys, allowing the antenna to be easily lowered for adjustment. I supported the pulleys from the top of the attic walls with a length of plastic rope, which also serves as an insulator. The antenna end insulators (not illustrated) must withstand high voltage in operation, so a bit of care must be taken with their design to insure that you don't start a fire in your attic! I fashioned mine by drilling holes at the ends of lengths of scrap plastic rod stock. A generous length of rope was attached to each end insulator, and screw eyes were used as pulleys to allow the antenna to be easily raised and lowered.

[*] The original sheet of Plexiglas® was purchased to replace a pane in the shack window so that holes could be easily drilled for the purpose of bringing cables into the shack. When I move from this QTH I can replace the original glass pane, leaving no trace of my antenna installation.

Choke Balun

I constructed a choke balun near the antenna center insulator by wrapping approximately 6 feet of the antenna coaxial-cable feedline as a single layer winding on a scrap polyethylene food container that was approximately 4 inches diameter. I used cable ties through small holes drilled in the container to secure the coax winding.

Some amateurs argue that a balun is not necessary when feeding a dipole with coax, but the proximity of this antenna to other objects and the physical constraints of attic installation make antenna symmetry unlikely in this situation. The simple choke balun used here is trivial to construct, and I do not feel it is worth the risk of feedline radiation problems to omit it.

Antenna Dimensions

The final dimensions of my antenna are shown below. If you try to duplicate this antenna you should start with longer lengths and then trim as necessary, as the lengths will be affected somewhat by height above ground, and in an attic installation by proximity to the building. An antenna analyzer, such as the MFJ-259 that I used, greatly speeds the trimming process.

If you are not interested in the 30 meter WARC band, here are the dimensions of the antenna without the 30 meter traps. You may note that the 80 meter end sections are significantly longer in the version without the 30 meter traps: much of that difference may be due to the larger percentage of the 80 meter section length that had to be bent to fit my attic in that version.

Electrical Measurements

One of the most often quoted disadvantages of trap antennas is reduced bandwidth. But the useful bandwidth of the coaxial trap dipole described here is sufficient for no-tuner use over much of the 6 bands. As the measurements in Table 2 illustrate, the antenna performs with better than 2:1 SWR over the entire 10 and 15 meter amateur bands. Almost all of 20 meters is usable with less than a 3:1 SWR. The 40 and 80 meter bands were trimmed for operation within the CW band segment.

Table 2. 2:1 and 3:1 SWR Bandwidth (Measured with MFJ-259 Antenna Analyzer)

amateur band	2:1 SWR	3:1 SWR
10 meter (28.0-29.7 MHz)	2.2 MHz	4.23 MHz
15 meter (21.0-21.45 MHz)	640 kHz	1.04 MHz
20 meter (14.0-14.35 MHz)	190 kHz	330 kHz
30 meter (10.1-10.15 MHz)	100 kHz	190 kHz
40 meter (7.0-7.3 MHz)	50 kHz	110 kHz
80 meter (3.5-4.0 MHz)	60 kHz	200 kHz

Table 3 contains the resonant frequencies and SWR, 2:1 SWR limits, and 3:1 SWR limits of the antenna as measured after the final trimming of each of the elements.

Table 3. SWR vs. Frequency (Measured with MFJ-259 Antenna Analyzer)

SWR	10 meter band	15 meter band	20 meter band	30 meter band	40 meter band	80 meter band
3	27.17 MHz	20.64 MHz	14.00 MHz	10.05 MHz	7.06 MHz	3.56 MHz
2	27.70 MHz	20.83 MHz	14.07 MHz	10.09 MHz	7.09 MHz	3.64 MHz
resonance	28.65 MHz @ 1.0 52 ohms	21.14 MHz @ 1.3 54 ohms	14.16 MHz @ 1.3 44 ohms	10.13 MHz @1.6 82 ohms	7.12 MHz @1.8 35 ohms	3.67 MHz @1.9 39 ohms
2	29.90 MHz	21.47 MHz	14.26 MHz	10.19 MHz	7.14 MHz	3.70 MHz
3	31.40 MHz	21.68 MHz	14.33 MHz	10.24 MHz	7.17 MHz	3.76 MHz

On-The-Air Performance

I finished installing this antenna on a Saturday. The next morning I connected my Heathkit HW-8 QRP rig and answered the first CQ I heard, which was an SM5 (Sweden) station on 15 meters. He responded to my call and we had a nice QSO, with solid copy on every exchange. I was running 2 watts output. I've had similar results on the other bands as well.

The performance of this attic coaxial-cable trap dipole doesn't compare to the 10-15-20 meter Yagi and 45 foot tower I enjoyed at my former QTH, but it continues to surprise me with just how well it does work. I have found the SWR bandwidth adequate for no-tune operation with my transceiver across the entire 10, 15, 20, and 30 meter bands, and the CW segment of 40 and 80 meters. I experienced no RFI problems at QRP power levels, but I did experience serious RFI problems with our stereo receiver at QRO (100 Watt) output power on 40 meters that I remediated by wrapping its power and surround speaker cables around split core "snap on" filter chokes (Radio Shack 273-104).

Your Mileage May Vary

Although many hams succeed with attic antennas, I know several who have tried attic dipoles and were disappointed with their performance. Perhaps my attic is more "antenna friendly" than theirs, or perhaps other factors conspired against them. I do hope that this story will inspire others with restrictive covenants (or restrictive spouses!) to not give up. This antenna has made it possible for me to operate a satisfying HF station in spite of the restrictive covenants imposed on my dwelling. Good luck, and I hope to hear you on the air soon!

References

[1] R. H. Johns, "Coaxial Cable Antenna Traps," QST, May 1981, pp. 15-17.
[2] G. E. O'Neil, "Trapping the Mysteries of Trap Antennas," Ham Radio, Oct 1981, pp 10-16.
[3] D. DeMaw, "Lightweight Trap Antennas -- Some Thoughts," QST, June 1983, pp. 15-18.
[4] R. Sommer, "Optimizing Coaxial-Cable Traps," QST, Dec 1984, pp. 37-42.
[5] J. Grebenkemper, "Multiband Trap and Parallel HF Dipoles -- A Comparison," QST, May 1985, pp. 26-31.
[6] D. Kennedy, "Coaxial-Cable Traps", QST, August, 1985, p. 43.
[7] M. Logan, "Coaxial-Cable Traps", QST, August, 1985, p. 43.

Frequently Asked Questions

Since posting this web page, dozens have written me with questions about my antenna or to report that they successfully constructed their own trap dipole after reading this paper. Below are the most frequently asked questions I have received:

• Can one add the 12 and 17 meter WARC bands?

  See Appendix - Why Aren't 12 and 17 Meters Supported? below.

• Can 80 meters be deleted? Can 80 meters and 40 meters be deleted?

  Yes. Remember that antenna traps become a high impedance at their resonant frequency, so the trap essentially becomes an insulator at resonance and everything after the trap is disconnected from the antenna.

  To delete 80 meters, simply omit the 40 meter traps and everything after them, placing the end insulators where the 40 meter traps used to be, and you'll have a 10/15/20/30/40 antenna. The dimensions of the remainder of the antenna will be unaffected except that the 40 meter segment lengths might need to be lengthened slightly due to the removal of the end loading provided by the 40 meter traps.

  Similarly, to delete both 80 and 40 meters simply omit the 30 meter traps and everything after them, placing the end insulators where the 30 meter traps used to be, and you'll have a 10/15/20/30 antenna. The dimensions of the remainder of the antenna will be unaffected except that the 30 meter segment lengths might need to be lengthened slightly due to the removal of the end loading provided by the 30 meter traps.

• Can this antenna be installed outdoors?

  Yes, however you may want to better weatherproof the connections, e.g. prevent water infiltration into the ends of the coax used for the traps and replace the wire nuts with soldered connections. [Note: I know one Southern New Jersey ham who made no effort to weatherproof his homemade outdoor coaxial cable trap dipole. He reports that it still works great after more than 10 years of exposure to the elements!]

  If you are in a region where ice or wind loading are likely you may also want to improve the mechanical strength, e.g. use more substantial center and end insulators.

• Can this antenna be used in a vertical orientation?

  Yes. For example, Gareth KH6RH constructed a 10/15/20 version of this antenna and hung it vertically in a tree near his appartment building. He wrote:

  "Not really having a good spot to hide a horizontal dipole for hf, vertically up in the tree works great. SWR is very reasonable, way under 2:1 where I stray. It's so nice to have 3 bands and no tuner on one antenna. I did have a single delta loop in the tree tuned for 10m, but after the tree trimming, it became too noticable."

• Can this antenna be used in an "inverted vee" orientation?

  Yes. Since it is electrically a half wave dipole on each band it will have a similar radiation pattern to a full size inverted vee hung at the same elevation. Below is an excerpt of an e-mail from KH6RH which I received several months after he first wrote about orienting a 10/15/20 meter version of this antenna vertically:

  "Not being one to sit still for long, I stared at my tree outback long enough to visualize I can mount the CTD [coaxial-cable trap dipole] horizontally, inverted vee style, and have it still "hidden" to the untrained eye. You know what, John, it works even better. I've been copying Europe, Asia, NA, SA, and Africa with this setup. I run the NCDXF beacon tracking program, and can hear the ZS6 beacon pretty much every day, on 20, 15, & 10m. Yes, the other QTH's come in too, but ZS6 being on the complete opposite side of the earth from KH6 makes it extra special. So you can imagine, hamming has been a lot of fun the last couple of months. Yes, the CTD does work vertically, but horizontally seems to work better, by the lobe of my ear, anyway. SWR is a tad higher on all 3 bands, but from what I've been hearing, it doesn't seem to affect performance."

• Why are the traps resonant at the center rather than at or below the lower edge of the band of interest, as in some other trap antenna designs?

  Modeling suggests that a fractional dB increase in gain is possible with a lower resonant trap frequency (e.g. see the discussion The Effect of Trap Resonant Frequency on Performance in http://www.cebik.com/trapg.html.) Unfortunately, this practice reduces the isolation provided by a trap at resonance which can make pruning the elements more difficult. It also raises the feedpoint impedance. I believe these disadvantages outweigh the insignificant theoretical improvement in antenna gain.

Appendix - Why Aren't 12 and 17 Meters Supported?

I did not include 12 and 17 meters in the series trap dipole described above because the loading effect of the 10 meter traps when operating on 12 meters would require the length of the 12 meter elements to be negative in order to achieve resonance. Similarly, the loading of the 15 meter traps when operating on 17 meters would require negative length 17 meter elements.

If operation on the 12 and 17 meter WARC bands is desired, one could construct a second trap dipole for those two bands using a pair of traps resonant at 12 meters. The inner elements (approximately 112 inches each) would form a full size half wavelength 12 meter dipole. The length of the outer (17 meter band) elements would be reduced by the loading effect of the 12 meter traps. In Table 4 below I include the dimensions of 12 meter traps that could be used in the construction of such an antenna. Table 4 also includes dimensions for 17 meter traps. These would not be used for a 12/17 meter dipole, but are included for completeness in case one is interested in constructing another band combination, e.g. a trap dipole covering the 12/17/30 meter WARC bands which would require a pair of 12 meter traps and a pair of 17 meter traps.

Table 4. Specifications of traps for 12 and 17 meter amateur bands

band	design frequency	trap form	coax length	coax turns
12 meters	24.94 MHz	1.375" OD (3/4" PVC coupling)	22.7"	4.4
17 meters	18.118 MHz	1.625" OD (1" PVC coupling)	29.2"	4.9

A 12/17 meter dipole could be fed with a second feedline or alternatively, it could be connected in parallel with the 10/15/20/30/40/80 dipole described in the main section of this paper. I have not tried or modeled the parallel connection so I do not know what interaction, if any, would occur between the two antennas.

Revision History

08 Sep 2003	Clarified trap connection illustration and added FAQ regarding trap frequency relative to band of interest per suggestions by Will W1ZRV.
26 Aug 2002	Add inverted vee testimonial and trap construction hints. Reverse trap form dimensions to emphasise OD rather than nominal size of PVC coupling.
24 Aug 2002	Add appendix regarding 12 and 17 meters, cutaway illustration of trap conections, paragraph comparing a trap dipole to a non-resonant antenna plus antenna tuner, paragraph on trap orientation, FAQ section, and other minor changes.
30 Nov 1999	Restore dimensions of antenna without 30 meter coverage.
06 Apr 1999	Added 30 meter coverage.
03 Jan 1999	Added 80 meter coverage.
10 Dec 1998	Original version for 10, 15, 20, and 40 meters.

Multiband Vertical Ground Plane

 6 Band "CB Special" Multiband Vertical Ground Plane
A quick and cheap 40 thru 10 meter vertical ground plane antenna!
by N4UJW
Do you have an old CB vertical laying around collecting dirt and mold?
Then you are in luck if you want to build a cheap "6" band HF vertical antenna!

:Getting Started collecting the "gold:
A couple of years ago, my XYL heard of a person that had some "antenna stuff" to give away so she was very reluctant to tell me knowing that I was usually not one to turn down a "good deal" when I saw it, but, long story short, she let the cat out of the bag. I drove about 5 miles or so and there it was, a "gold mine" of aluminum tubing just laying there to be panned.

Among the pile of "gold", I spotted the base of what appeared to be an old "base station" style CB antenna, you know, the ones that were about a half wave or so long on 11 meters and had those fancy sounding DX grabber names!

Not only did it have the original insulated base and all that went with it, but there buried under all that metal was the working end of it, all that aluminum telescoping tubing! I managed to load up ALL of that "antenna stuff" on top of and inside the car and proceeded home trying to evade the local "county mounties". Enough of that CB talk! I am supposed to be a ham.

How I did it:
Well, longer story shorter, here is how I converted that old CB relic into at least a 5 bander vertical ground plane for use on the hf ham bands. Yes I know, I said 6 bands earlier, this may depend on your setup and tuner. No, you can't get something for nothing. A decent tuner will be required for this project.
(I used the MFJ 901B).

Now really getting started with the help of "Murphy's Law":
The first thing I had to do since it had been laying out behind my house for a couple of years, was to get it apart and do some cleaning of all of the tubing where the sections went together. The screws were extremely rusted and they were a bit difficult to get out of the tubing sections. Then as Murphy's law would have it, the tubing sections were stuck together; so with the aid of a pair of good Channel Lock type pliers, apart they came except for the bottom section that went inside the loading coil?/matching portion of the base.

With much effort, I was able to get the outer case "protecting" the coils inside, pushed up toward the mounting bracket just enough to see the coils.
Murphy stepped in...the coil cover had not done it's job!

The coils were in very bad shape having been subjected to years of rain, dirt, corrosion, etc....so I just took them out of the circuit by clipping a couple of the wires. At the very bottom of the base of the coil cover, there was an SO-239 connector with a nut attached. The nut would not budge much and the threads were striped in the process of my trying to loosen it....junk.

I then used an ohm meter to make sure that there was no continuity between the center of the SO-239 and the upper aluminum tubing or to any other part of the existing aluminum or mounting bracket...there was none, so the entire coil assembly in the base was now "out of the circuit" so to speak and I was free to use ONLY the tubing of the old antenna for the vertical radiator of this project which was my original intent anyway.

The next step was to clean the area of the tubing where the radiating sections went together with some fine grade sandpaper down to bright aluminum. This went well and did not take long. murphy must have been taking a break!

About the length of the vertical and the radials:
From ideas on other projects like those of KL7JR and others on this site and on the web, I had learned that to make this antenna work well on many HF bands...the radiating portion of the antenna was to be of a non-resonant length on any ham band.
 
I did some more research on the web and determined that the length of 15 feet for the vertical radiator should work well when the radiator was fed against a few radials of the same or longer length and with a tuner. I used 3 radials of equal length of about 15 feet.

So the next step was to put the telescoping sections back together, adjust the length to about 15 feet from the now non-existing feed point near the top of the mounting bracket and secure everything with screws. I had sort of reversed the procedure by not having a feed point ready, and this is when I saw murphy looking over my shoulder again! He laughed and walked away!

Now I had to come up with a feed point on the bottom section of the tubing near the base so I could get the radiator length right....get away murphy!
I wanted to feed the antenna with coax so now I had to come up with a way to connect the PL-259 from the end of the feed line to a non-existing SO-239 that I wanted to use!
As luck would have it, there hidden within the pile of "gold" aluminum, was an old bracket from another "junker" antenna that contained not one one, but two! Murphy must now be on vacation!

I cleaned the old bracket containing the SO-239 with fine grade sandpaper and also gave the SO-239 a good cleaning inside and out with an assortment of small steel wire brushes, then it was a simple matter of just drilling a small hole or two for a couple of small bolts to mount the connector bracket to the original mount near the future feed point on the vertical portion of the radiator. I won't mention here that Murphy came back from vacation and proceeded to scatter nuts and bolts everywhere!

The hookup to the SO-239:
The "tip" end of the connector was facing up toward the top of the radiator just like I wanted it so it was a simple matter of eyeballing the length of wire needed to connect the center pin tip to the vertical section only a couple of inches away. Oooops...now I've to drill another hole for the wire to wrap around a screw on the radiator.

When I was finished drilling and adding the screw, connecting the wire from the connector to the radiator was a simple process that Murphy did not interrupt.

Now I was finished......but that darned Murphy was still around laughing at me!
I looked over my new prize of a 5 or 6 band hf vertical from top to bottom and wondered what I had forgotten.....there must be something...the way he is laughing!

Yep, sure enough.....I had forgotten to mechanically secure the sections together with screws! Upon lining up all of the holes in the several sections of tubing, I realized that all would not match up......DARN! I looked over my shoulder.....you know who.... was right behind me....DRATS! Out came the drill again....I finally found some screws that should be OK to use and the correct size drill bit, so I drilled the holes in each section for them, then  added the screws and tightened them......

I must be finished now! You guessed it.....the radials were missing and Murphy was holding the wire cutters and the tape measure.

Finally after measuring out 3 equal length radials of about 15 feet each and getting them cut without "incident", I mounted them in three fairly equal distance locations around the mounting bracket just below the feed point!

Up she goes!
I now had a completed multiband vertical that had to be mounted on a short mast and put up beside the house . No problems there other than having to use new "U" bolts on the mounting bracket and tie off the radials on the ends. They were at about a 30 degree angle. Don't forget to connect the feed line Don!

I used two lengths of 50 ohm coax, one section about 25 feet long of RG8 connected to the antenna base, the other was about 10 to 15 feet of RG58.....giving me a total of about 35 feet or so of feed line to the tuner.

Swr and resonance testing without Murphy:
Using an MFJ 259B these are the results at the end of the feed line below.
7.200mhz         15:1 swr
10.100mhz      7.3:1 swr
14.275mhz      2.3:1 swr
16.216mhz         1:1 swr (R=55, X = 0)
21.350mhz      3.7:1 swr
24.960mhz      5.2:1 swr
28.400mhz      4.4:1 swr

ON THE AIR PERFORMANCE. The proof is in the pudding!
Over a few days of on the air testing in the low part of the solar cycle in 2008,
 I have managed to make several contacts every time I operated with at least 5 8 reports, with most of them being 5 9 or better on 40, 20, and 10 meters using 100 watts and the tuner.
The 40 meter contacts were late in the day to the East coast.

The 20 meter contacts were late afternoon.
 
15 meters has been dead here for some time but I managed to make a schedule with a local ham some distance away, who gave me a good report on a test transmission.
10 was "dead" at the time of testing but I received a nice report from the same local station several miles away that usually only could read me about S5 or S 6 on a multiband doublet, but received me much stronger on the vertical. I could hear him also much better when compared to my Hustler 4BTV or multiband doublet. He was using an Inverted V if memory serves.

DX results.....TA3D  !!!!!!
The antenna and the YAESU FT-107 running 100 watts really shined on 20 meters during this contact. I managed to break a HUGE pile up with him and received the usual 5 9 report in TURKEY!

So...... I guess it "works", but so does a paper clip to some extent! This "paper clip" just happens to work much better!

Final comments and suggestions:
My overall impression of the CB antenna converted to use on many hf bands using a tuner has been a pleasant and rewarding project for me and the good thing about it was the fact that I only had TIME invested in it and a little wire, a few screws plus dealing with Murphy while I used other peoples "junk" to make a multiband hf vertical antenna that worked around the globe, coast to coast and locally!!

When using the MFJ 901B tuner, the "6 Band CB Multiband Vertical Ground Plane tunes up nicely on ALL ham bands from 40 thru 10 meters giving a great match of at or near 1:1 swr on all the hf bands from 40 thru 10 meters.
It also allows me to "hear" 80 meters as if I had an 80 meter dipole up. Also to my surprise, it does not seem to be as noise prone as most verticals I have tried or maybe my noise level has just been low lately. 
I look forward to more on the air testing as time progresses.
So to repeat my self, I guess it works!

Below is a drawing with lengths that I used and the location of connector bracket:

Total height about 27 to 30 feet to top

Modified SO-239 location and bracket on base mount.
Attach radials to any suitable point just below feed point.
Main vertical radiator is totally insulated from mounting bracket at both
 the top side and the bottom although not shown in drawing.

That's it....enjoy....,
 and save some room in the rf spectrum for me....73 Don, N4UJW

A Practical Antenna for 160 Meters

The Broomstick Special

A compact, easy-to-build shortwave antenna

By Arnie Coro
Host of "Dxers Unlimited", Radio Habana, Cuba

This is a "helically wound" wire antenna. It can be built in a few hours... it will take longer if you really want to make it look pretty... ugly versions can be assembled in minutes. It may be used without an antenna tuner, but... it works best when you do use a tuner.

Here are the easy-to-follow, step-by-step building instructions for Arnie Coro's "Broomstick Special":

1.
Obtain a nice broomstick.... you can use a "classic" wooden dowel broomstick, or substitute heavy walled PVC plumbers plastic tubing

2.
Diameter of broomstick is not critical; anything from about 1.5 centimeters or better will work (this means that PVC tubing of about 19mm or 3/4 inch is ideal).

3.
Prepare a base to hold the broomstick or PVC pipe vertical... Use a wide base, with enough counterweight attached to keep the broomstick vertical (I use mine next to the bedside radio, have convinced the wife that it is "modern art"

4.
Obtain an aluminium disk of no less than 15 cm (6 inches) diameter. I prefer using a disk of around 30 centimeters (12 inches) but this is not critical. DO USE THE DISK... as it provides a capacity hat termination and helps reduce NOISE PICK UP

5.
Obtain enough No.16 PVC plastic covered household wire; this is the ideal choice, but if you can't find it, then you may use No.16 or No.18 enamel covered copper wire (the one used for winding motors and transformers). If you can't find No.16 PVC covered wire, then your second best choice is No.18 "speaker wire"

6.
Connect one end of the wire to the aluminium disk, and start winding a uniform coil using the "broostick" as the coil form. Yoy want a neat job! Turns should fit tightly one next to the other... the "broomstick" will be filled with the wire forming the coil... When you arrive at the bottom end, make a termination> I use a long bolt with nuts and washers to which I tie the end of the wire, and another wire that goes to the antenna tuner. This wire that goes to the tuner can be from 1 meter to 3 meters long (from 3 to 10 feet) but do keep it as short as possible.

7.
After the antenna is built, you may want to protect it with tightly wound PVC plastic tape over the wire. For extra protection, you can paint the whole antenna with several coats of spray enamel...



8.
The antenna works best near a window. Or better yet, you can install it in your balcony or garden... but do keep the connections to the tuner short.

9.
The antenna is resonated with your antenna tuner.... you may use it without a tuner, but results are not going to be as good as when the antenna is connected to the receiver via a well-designed antenna tuner

10.
Yes... you may use it for transmitting.... but... according to recent medical research information, keep it as far away as possible from your body!

11.
The helically wound "broomstick" is a lot of fun to experiment with... It works best when you provide a ground connection to the antenna tuner - receiver combination. One way of providing an "artificial ground" is to connect a length of wire of no less than 5 meters as a "counterpoise," that meaning that you can let the wire hang around the room's floor or garden. Using the "broomstick" with resonant radials turns it into an excellent amateur radio antenna for a specific band... For example with 4 radials cut for the 15 meter amateur band and a 4 feet high broomstick, (about 1.5 meters)

I can work a lot of stations on 21 mHz, something I do often to demonstrate to visitors what can be achieved with simple homebrew antennas, even when you don't have a lot of space.

The Broomstick Special, Part 2

By Arnie Coro
Host of "Dxers Unlimited", Radio Habana, Cuba

A few more tips on how to optimize this simple linear mode helical antenna:

If you have built and tested my "broomstick" helical linear mode antenna, using the data supplied in the first article, you should have noticed that:

A.
The antenna performs quite well even without an antenna tuner

B.
When used with even the simplest LC tuner in an L network configuration, the "broomstick" is quite competitive with any other indoor antenna.

C.
You have by now realized that this is about the cheapest and easier to build antenna!

Why Part 2 then? Well, because there is always room for improving an already good design...

Many years ago amateur radio magazines carried a rather small ad about a so-called Joystick VFA antenna... made in England. With the ad came some rather amusing claims about DX worked... which happened to be true! The Joystick VFA was nothing more than a helically wound antenna... and it worked!

The "Broomstick" is a little bit more simple... but as I said, you can improve it in the following ways:

1.
Install a counterpoise or ground system. This may be as simple as a single wire coming out of the antenna tuner and extending from 5 meters to 20 meters ... it need not be installed in a linear fashion, I have used this counterpoise system with the wire tacked to the wall around the room!

2.
Resonate the antenna's main helical loading plus its extension to the operating frequency, something that requires patience, a Grid Dip Meter, more patience, and then a good, high quality standing wave ratio meter.

3.
Install the antenna outside, and connect it to the antenna tuner with a single wire feedline made of no 14 or no 12 copper wire... You want this wire short, at least as short as possible, and you must be really careful how it travels from the base of the Broomstick to the antenna tuner. You want to avoid a bad RF burn if the antenna's lead in touches someone's skin!

4.
If you choose no 3 approach, then you must resonate the combination of single wire feedline plus Broomstick to the operating frequency, something that may turn to be especially critical when using the antenna for transmitting, but that is not so critical if you are using it for receiving only!

5.
Broomstick antennas are excellent club projects, as they can be mass produced at very low cost... especially if you buy materials in quantity. The use of PVC plumbers pipe, the white colored stock, works quite well with transmitters in the 100 watt class, but don't use it if you ever intend to run higher power. High power - i.e. more than 100 watts should be avoided with the Broomstick!

A Typical Installation

A "Broomstick" antenna made with a 2-meter section of 25-mm diameter plumbers PVC pipe, terminated with a 30-cm diameter top loading disk is the minimum I would use for transmitting. Ideally you should try to use no less than 2.5 meters, but many modern buildings will not be that high ! Please do remember to use the antenna for transmitting only if it can be safely placed, as far as possible from you and any other persons or animals in the room!

The helical distributed loading can be optimized a bit by winding it with different pitch.... starting with turns separated about 2 wire diameters at the base, going to one wire diameter separation at the middle, and ending with closely wound turns at the top of the antenna, were it connects to the aluminium top capacity loading disk.The general rule to follow is to wind enough wire so that it makes a half wavelength at the lowest operating frequency you want the antenna to work... Example... for a "Broomstick" designed to operate from 7 MHz up, the wire length should be no less than 70 feet. It's always a lot easier to remove a few turns at the top, than to have to splice the wire and then add a few more turns!

Yes, the distributed loading works better, but it is really difficult to tell the difference when using the "Broomstick" in actual practice!

About tuners!

A simple PI network tuner works very well. For RO (receive only) I use two old capacitors from defunct vacuum tube radios, connecting only the 365 pF sections.

The coil is made also from PVC pipe... I use a 40-turn coil wound on a 25-mm diameter PVC pipe, I tap the coil every 5 turns, and use an alligator clip to select the best tap for the frequency I want to operate. You can add a wafer switch (ceramic is best if transmitting) to select the taps at the coil... a 10-position switch is ideal, but you can do well with a 6-position one, too.

Specifications of common coax